The Mitogen Activated Protein Kinase/ Extracellular signal Regulated Kinase (MAPK/ERK) cascade controls cell growth and survival and adhesion. I have used an expression-cloning strategy to isolate a protein that blocks ERK/MAPK (p42/p44) signaling, PEA-15. PEA-15 impairs several ERK functions such as activation of transcription. PEA-15 binds directly to ERK1/2 and co-localizes with it in the cytoplasm. While PEA-15 contains a death effector domain (DED) it affects apoptosis (programmed cell death) in only a subset of cell types, specifically some fibroblasts and astrocytes. In the PEA-15 null mouse, the most striking phenotype is a lymphoproliferative syndrome affecting both B and T cells. There is also a marked increase in hepatocyte proliferation and atrophy of the epidermis. PEA-15 maps to human chromosome 1q21. This is a gene rich locus implicated in a number of diseases, including acute myeloblastic leukemia and the skin disorder Ichthyosis vulgaris. PEA-15 has been cloned as a gene upregulated in patients with type II diabetes and appears to affect glucose metabolism in an undefined manner. Finally, the 3' untranslated region of PEA-15 has been cloned as a proto-oncogene, MAT-1, isolated from breast cancer cells. Hence, understanding the mechanism, of PEA-15 regulation of ERK signaling may have direct clinical relevance. The specific aims of this project are to: (1) Determine the residues of PEA-15 that are necessary for ERK binding by mutational analysis; (2) Determine the molecular mechanism by which PEA-15 affects ERK signaling; (3) Analyze the expression of PEA-15 mRNA and protein; and (4) Examine the function of PEA-15 expression on cell behavior in both cultured cells and cells derived from PEA-15 null mice. This work will illuminate the functional significance of PEA-15 and the molecular mechanism by which it acts. At the conclusion of these studies we will better understand how alterations in cell adhesion, migration, survival, and proliferation can be regulated by a novel player in cell signaling events. Moreover, this work will elucidate the potential role of PEA-15 in the pathogenesis of cancer and diabetes.